Often, developmental stages are defined by milestones. A milestone is a sort of marker that tells you where you are while traveling. The term is drawn from literal stone markers that were used to mark the passage of each mile on early roads. Today, the term milestone is used more figuratively, to indicate that a developmental stage has been achieved. Often, special milestones mark children's accomplishments, such as walking in infancy and entering school in early childhood, and these milestones can help mark children's movement inside and between developmental stages.
Children build new skills and developments on top of old skills and developments from stage to stage; each stage is cumulative. A child is able to run bases in a game of baseball in the middle childhood phase because she was first able to walk near the end of her infancy stage.
Entry and exit from the various developmental stages tends to occur at particular ages. Often, a child's stage of development can be figured out by a child's age because children generally experience the same stages at the same ages. However a child's age only provides a clue as to his stage; it does not determine it. Every child develops at his or her own speed. It is the tasks and skills children master that truly identify what stage they are in. Because of this, different children of the same age can be expected to be at different developmental stages.
Children's development does not happen uniformly, but rather, it progresses along at its own rate. Just because one child is potty trained at age three and his neighbor is potty trained at age three and a half does not mean that one is brighter than the other. Furthermore, children can develop the different channels at different rates. For example, a twelve-year-old's body may have already gone through puberty and look like an adolescence's body, but that child may not have the cognitive and social abilities of an adolescent quite yet. It will take a little longer for their mind to catch up with her body.
Keep this lack of developmental sameness in mind as you read the documents in this series. Whenever a document suggests ages that children reach specific milestones, keep in mind that these are general average ages that research has found children develop these skills. In reality, children reach milestones across a wide range of ages. Sometimes children will appear to even skip an entire developmental stage in some channels as they advance quickly in a short amount of time.
Also keep in mind that there are some situations in which children become severely inhibited and unable to reach certain milestones within an acceptable time frame. Developmental delays in a child's functioning caused by disease, injury, mental disability, problems developing in the womb, environmental reasons, trauma or unknown causes can keep some children from developing properly or can even cause children to regress and go backward into some stages in some channels.